With rapid progress of robust intelligent power grid, a super-large AC synchronous power grid is gradually shaped up after grids networking in East China, North China and Central China. As backup and supporting system of high capacity DC ultra-high voltage power transmission system, the AC extra-high voltage greatly strengthens interconnection of power systems in different regions. The expansion of AC grid scale results in increase of equivalent inertia of overall power system, thus leading to increasingly low frequency oscillation of the system. In addition, it is inevitable for the remote heavy load AC transmission to cause reduction of system damping and increase of risks in low frequency oscillation and dynamic stability in super-large grids. The power system stabilizer is still the most effective and most economical means to damp low frequency oscillation at present. At the backdrop of super-large AC synchronous grids, PSS is required to cover 0.1 Hz low frequency oscillation suppression.
The current PPS that is most widely used is PSS2B power system stabilizer having good high frequency oscillation suppression, liability to field setting test and anti-reversal suppression.
See FIG. 1 for PSS2B model. To form accelerating power by overlapping active power signal Pe and speed signal ω, PSS2B adds Class 1 Order 1 inertial element T7 and gain factor Ks2 after DC blocking element of active power, causing 90° lag of phase characteristic of power fluctuation signal for PSS2B after DC blocking. Since the lag degree caused by Class 1 Order 1 inertial element can meet the requirement of phase compensation by at least Order 2 lead of three lead and lag elements provided by PSS2B, the three-machine brushless excitation even uses three orders of lead compensation to meet the requirements of phase compensation specified by relevant standards in 0.1-2 Hz full band. Capable of well fulfilling phase compensation in 0.1-2 Hz band, Multi-order lead compensation will cause sharp increase of high-band gain, thus limiting amplification factor KS1 of PSS2B actual setting. Limiting of PSS2B high-band gain results in insufficient low-band gain, which severely weakens its oscillation suppression ability in intermediate and low frequency.
See FIG. 2 for phase relation between existing PSS2B phase compensation and input signal. The input signal of PSS2B phase compensation element Vout1 approximates to speed signal Δω, so anti-reversal is suppressed by compensating active signal (which enables change of mechanical power) and speed signal. The lag of uncompensated lag characteristic phase of generator measured according to theoretical analysis and engineering is less than 90° in intermediate and low band, but generally above 90° in high band. Both low-band output Vpss low and high-band output Vpss high of PSS2B adopt lead compensation. However, the gain of lead element quickly increases along with frequency, directly causing that PSS2B fails to solve harmonious configuration of phase and gain in full band.
Now, the field PSS test is verified by load voltage step, and the disturbance waveform caused is at oscillation frequency point of the machine, only belonging to high band of low oscillation frequency. Therefore, the field step disturbance test shows good oscillation suppression effect of PSS2B, but its low-band oscillation suppression effect is hard to be guaranteed.
On the other hand, currently most of oscillation frequency of AC synchronous grids is in intermediate and high, for example, the possible minimum oscillation frequency of independent East China Power Grid is about 0.5 Hz. Therefore, the problems concerning insufficient suppression in low frequency of PSS2B are not evident. However with formation of extra-high voltage large AC synchronous power grids, the frequency for low frequency oscillation of the system is increasingly low. Sufficient positive damping is required to provide at 0.1-2 Hz of PSS by the industry at present, and the problems of insufficient gain in low-frequency band of PSS2B emerge.
In order to meet the suppression requirement in low-frequency band, the research of multi-band PSS booms in recent years, and various new PSS models have been proposed. However, all have weakness of unclear physical conception and difficult field setting and computation, so they haven't been used in any project.
Currently, the power system stabilizer which has better suppression ability in high and low-frequency bands as well as simple and feasible field setting method with clear physical conception both at home and abroad is in shortage, making super-large power grids have reduced damping level, sufficient suppression ability and dynamic stability problems in low band.